This invention relates to a blood filter for removing foreign matters and bubbles from blood passing an extracorporeal blood circuit including an artificial heart-lung or pump-oxygenator, an artificial kidney or dialyzator, and a device for separating blood cell and plasma components, and more particularly, to a blood filter having improved debubbling capacity.
A number of blood filters are known in the art. One typical blood filter is disclosed in U.S. Pat. No. 4,411,783. In FIG. 14 of the present drawing, the blood filter generally designated at 100 is illustrated as comprising a cylindrical housing 102 formed of polycarbonate resin, for example, having a blood inlet 105 and a blood outlet 108, and a filter member 109 received in the housing 102 between the inlet 105 and the outlet 108. The filter member 109 is prepared by sandwiching a mesh screen with an opening of 20-50 .mu.m between plastic nets, folding the sandwich in pleats, and mating the ends to form a generally cylindrical filter member. To prevent bubbles from directly passing to the filter member 109, the blood inlet 105 is tangentially connected to the cylindrical housing 102 such that blood B may enter the housing interior in a tangential direction to form a swirl flow therein as shown in FIG. 15 which is a cross section of the blood filter of FIG. 14 taken along lines III--III. It is also known to interpose a continuous foam between the blood inlet and the filter member to prevent bubbles from directly passing to the filter member.
In the blood filter of the above-mentioned type wherein blood B enters the housing 102 through the inlet 105 to form a swirl flow therein, bubbles are removed by virtue of a centrifugal effect that fine bubbles with a small mass entrained in the swirl flow of blood B will collect toward the center of swirl. Since buoyancy applies at all times to bubbles in blood B, separated bubbles will float and collect at an axial top portion of the housing 102 and outflow through a vent 106 at the axial top portion of the housing 102. The filter member 109 removes only those bubbles which are not removed by the buoyancy and the centrifugal force of the swirl flow, that is, bubbles which are not entrained in the swirl flow of blood B. In the above-mentioned modification wherein a continuous foam is present between the inlet and the filter member, the foam removes such bubbles.
In the blood filter 100 mentioned above, however, the inflow of blood which is entering the housing 102 through the inlet 105 directly impinges against a swirl flow of blood which is already whirling in the housing 102 as shown in FIG. 15. Since this causes a disturbance in the blood inflow before the inflow merges with the swirl flow, bubbles are not so smoothly conveyed to the swirl flow of blood. Thus, this blood filter 100 has the drawback that much bubbles reach the filter member 109 because bubbles of a small mass cannot be so effectively separated from blood B by entraining bubbles on the swirl flow of blood and permitting such bubbles to collect toward the swirl center by virtue of a centrifugal effect. Bubbles in blood B impinge against and stick to the filter member 109 and some bubbles then gradually penetrate through the filter member 109 partially under the influence of a pressure variation caused by pulsation of a feed pump. Also, the above-mentioned blood filter of the type using a continuous foam has several problems including adherence of platelets to the foam, damage to cells, an increased pressure loss, and difficulty of debubbling upon priming.